1. Field of the Invention
The present invention relates to a resin-filled ferrite carrier core material and a ferrite carrier, used in a two-component electrophotographic developer used in apparatuses such as copiers and printers, specifically relates to a resin-filled ferrite carrier core material for an electrophotographic developer, a ferrite carrier and an electrophotographic developer using the ferrite carrier, wherein the electrophotographic developer is capable of maintaining a high charging ability for a long period of time while the advantages of a resin-filled carrier are being maintained, is capable of attaining a high image quality and is capable of reducing image defects.
2. Description of the Related Art
An electrophotographic development method is a method in which development is performed by adhering the toner particles in a developer to the electrostatic latent image formed on a photoreceptor, and the developer used in such a method is classified into a two-component developer composed of toner particles and carrier particles and a one-component developer using only toner particles.
As a development method using, among such developers, a two-component developer composed of toner particles and carrier particles, previously a method such as a cascade method has been adopted, but currently a magnetic brush method using a magnet roll predominates.
In a two-component developer, the carrier particles serve as a carrying substance to form a toner image on the photoreceptor in such a way that the carrier particles are stirred together with the toner particles in a developer box filled with the developer to impart an intended charge to the toner particles, and further, convey the thus charged toner particles to the surface of the photoreceptor to form the toner image on the photoreceptor. The carrier particles remaining on a development roll which holds a magnet again return from the development roll into the developer box to be mixed and stirred with the fresh toner particles and to be repeatedly used for a predetermined period of time.
In contrast to a one-component developer, a two-component developer is such that the carrier particles are mixed and stirred with the toner particles, thus charge the toner particles, and further have a function to convey the toner particles, and a two-component developer is excellent in the controllability in designing developers. Accordingly, two-component developers are suitable for full-color development apparatuses required to offer high image quality and for high speed printing apparatuses required to be satisfactory in the reliability and durability in image maintenance.
In two-component developers used in the above-described manner, the image properties such as the image density, fogging, white spots, gradation and resolution are each required to exhibit a predetermined value from the initial stage, and further these properties are required to be invariant and to be stably maintained during the endurance printing. For the purpose of stably maintaining these properties, the properties of the carrier particles contained in the two-component developers are required to be stable.
As the carrier particles which form two-component developers, there have hitherto been used iron powder carriers such as an iron powder carrier in which the surface of an iron powder is coated with an oxide film or an iron powder carrier in which the surface of an iron powder is coated with a resin. Such iron powder carriers are high in magnetization and also high in conductivity, and hence have an advantage that images satisfactory in the reproducibility of the solid print portions thereof are easily obtained.
However, the true specific gravities of such iron powder carriers are as heavy as about 7.8, and the magnetizations of such iron power carriers are too high. Accordingly, the stirring and mixing of such an iron powder carrier with the toner particles in the developer box tend to cause the fusion bonding of the toner-constituting components to the surface of the iron powder carrier, namely, the so-called toner spent. The occurrence of such a toner spent reduces the effective surface area of the carrier, and the triboelectric charging ability of the carrier in relation to the toner particles tends to be degraded.
Additionally, in the resin-coated iron powder carrier, the resin on the surface is exfoliated by the stress at the time of endurance operation to expose the core material (iron powder) which is highly conductive and low in dielectric breakdown voltage, and accordingly the charge leakage occurs as the case may be. Such charge leakage breaks the electrostatic latent image formed on the photoreceptor, causes brush strokes or the like to occur on the solid print portion, and makes it difficult to obtain a uniform image. Due to these reasons, currently the iron powder carriers such as oxide-coated iron powder carriers and resin-coated iron powder carriers have become decreasingly used.
In recent years, in place of the iron powder carriers, ferrites each having a true specific gravity of as light as about 5.0 and being low in magnetization have been used as carriers, and resin-coated ferrite carriers in each of which the surface of the ferrite is further coated with a resin have been frequently used, and accordingly the operating lives of the developers have been dramatically extended.
Recently office networking has been promoted, and the age of monofunctional copiers develops into the age of multifunctional copiers; the service system has also shifted from the age of the system such that a contracted service man conducts periodic maintenance inclusive of the replacement of the developer to the age of the maintenance-free system; thus, the market has further enhanced demand for further longer operating life of the developer.
Under such circumstances, for the purpose of reducing the carrier particle weight and extending the developer operating life, there have been proposed a variety of magnetic powder-dispersed carriers in each of which magnetic fine particles are dispersed in a resin.
Although such magnetic powder-dispersed carriers are light in true specific gravity and advantageous for the purpose of extending the carrier operating life, such magnetic powder-dispersed carriers tend to be high in resistance, make it difficult to easily attain intended image densities and disturb the charge amount control. Additionally, the magnetization control is performed on the basis of the amount of the dispersed magnetic powder, and hence it is difficult to establish a compatibility between the true specific gravity (an index for the operating life) and another physical property (image property control). Yet additionally, the magnetic powder-dispersed carriers are prepared by agglomerating magnetic powders with resins, and hence are low in hardness and tend to crack and tend to undergo magnetic powder detachment, thermal fusion and thermal deformation.
As a substitute for the magnetic powder-dispersed carrier, there have been proposed a resin-filled carrier in which the voids in a porous carrier core material are filled with a resin. The resin-filled carrier is prepared by forming extremely porous core material particles under suppression of crystal growth due to the control of the composition and the sintering (firing) conditions and by filling the resulting porous core material particles with an optional resin, and hence the filling of the porous core material particles with a resin enables to reduce the true specific gravity and to attain a long operating life. Additionally, by selecting the resin to be filled, the charge amount and the like can be easily controlled. As compared to the magnetic powder-dispersed carriers, the resin-filled carriers have advantages such that the resin-filled carriers are higher in strength and are free from the occurrence of the cracking, deformation and melting due to heat or impact.
However, a Mn-containing ferrite is low in electric resistance, and hence the dielectric breakdown voltage of the Mn-containing ferrite cannot be sufficiently increased even when the Mn-containing ferrite is filled with a resin, and thus offers a cause for image defects such as white spots. Additionally, when a ferrite is made to have such porousness that allows the filling of an intended amount of a resin, it is necessary to produce the porousness (pores) under the conditions that the sintering is not yet made to completely proceed by setting the sintering temperature of the ferrite at a low temperature. When the sintering is performed at such a low temperature, the inclusion of Mn in the ferrite tends to cause the generation of particles having low magnetization and tends to offer a problem such that the porousness (pore conditions) is different among the particles.
Further, it is desired not to use heavy metals such as Cu, Zn, Ni and Mn from the viewpoints such as the recent environmental load reduction and the occupational safety and health.
Accordingly, there has been proposed a carrier using a Mg-containing carrier core material. Specifically, Japanese Patent Laid-Open No. 2007-218955 discloses a carrier particle obtained by coating with a resin a carrier core material being mainly composed of Mg and/or Mn, having a magnetic phase in which the amount of MgO and/or MnO is 0 to 50 (molar ratio) and a nonmagnetic phase including one or more of SiO2, Al2O3 and Al(OH)2, and having a pore volume of 0.03 to 0.15 ml/g; an example discloses a carrier using a carrier core material obtained by mixing Fe2O3 and Mg(OH)2 in a ratio of Fe2O3:Mg(OH)2=80:20.
The content range of MgO and/or MnO disclosed in the above-described Japanese Patent Laid-Open No. 2007-218955 is extremely wide; it is impossible to satisfy both of the intended pore volume and the intended magnetic property over the whole of this range. In particular, in the case where Mn is not contained and in the case where the amount of Mg is small, when the sintering is performed at a sintering temperature set at a low temperature for the purpose of increasing the pore volume, a high pore volume is obtained, but at the same time, the magnetization is degraded. Additionally, in the sintering temperature range where the intended pore volume is obtained, the variation of the magnetization in relation to the sintering temperature is large to remarkably degrade the production stability and the development reproducibility of magnetization.
Japanese Patent Laid-Open No. 2006-317620 discloses a carrier powder for an electrophotographic developer wherein the carrier powder has as the core material thereof a ferrite particle that uses as a MgO source a Mg(OH)2-containing substance and contains MgO in a content of 5 to 35 mol %. When a ferrite is produced in such a composition range, it is possible to form dimples, for obtaining resin exfoliation strength, on the surface of the ferrite particles as described in Japanese Patent Laid-Open No. 2006-317620, but it is impossible to obtain a resin-filled carrier in which pores are proactively formed in the core material, a resin is filled (impregnated) in the pores and thus the weight reduction is achieved. In particular, in the case where X is less than 25 mol % in the formula (MgO)x(Fe2O3)100-x, when the sintering is performed at a sintering temperature set at a low temperature for the purpose of increasing the pore volume, a high pore volume is obtained, but at the same time, the magnetization is degraded. Additionally, in the sintering temperature range where the intended pore volume is obtained, the variation of the magnetization in relation to the sintering temperature is large to remarkably degrade the production stability and the development reproducibility of magnetization.
Japanese Patent Laid-Open No. 2008-107841 discloses a carrier for an electrophotographic developer wherein a core material has, as the constituent components thereof, iron, oxygen and magnesium and contains magnesium in a content of 0.5 to 10% by weight, and the core material is coated with a resin. When the above-described magnesium-containing ferrite is described with the general formula of the ferrite, it is meant that X is 2.3 to 33.8 mol % in (MgO)x(Fe2O3)100-x Japanese Patent Laid-Open No. 2008-107841 does not take as its object the increase of the void volume of the particles, as described in the paragraph [0010], and relates to a resin-coated carrier in which a ferrite core obtained by sintering at a somewhat high sintering temperature is coated with a resin, and the resin-coated carrier is completely different from a resin-filled carrier obtained by filling a resin in the pores obtained in the particles that are made proactively porous. Therefore, it is impossible to stably obtain the intended pore volume and the intended magnetic properties in the whole range of the Mg content extending over such a wide range as described in Japanese Patent Laid-Open No. 2008-107841.
Japanese Patent Laid-Open No. 2008-96977 discloses a carrier prepared by coating with a resin the surface of core particles composed of a ferrite containing at least magnesium element, wherein the maximum grain size on the surface of the core particles is 2 to 5 μm. Additionally, Japanese Patent Laid-Open No. 2008-96977 describes that the ferrite core particles containing manganese element are preferable, and in the paragraph [0058] thereof, there is a description that the proportion of Mg(OH)2 is preferably 10 to 40 mol %. There is also a description that in such a core material, the grain size is comparatively small, and by making the grain size fall within a specific grain size range, the surface of the core particles is made to have uniform roughness to consequently facilitate uniform coating of the core particles with a resin. It is also disclosed that it is difficult to make the grain size less than 2 μm in the production of the core particles. The ferrite described in Japanese Patent Laid-Open No. 2008-96977 is substantially a Mn-containing ferrite, and suffers from problems such that the electric resistance of the core material tends to be low and the variations of the magnetization and the porousness (pore formation condition) among particles tend to occur. The use of a heavy metal is also ineffective from the viewpoints such as the recent environmental regulation and the recent reduction of environmental load. Further, the proportion of Mg(OH)2 falling over a range as wide as 10 to 40 mol % makes it difficult to stably obtain the intended pore volume and the intended magnetic properties.
On the other hand, Japanese Patent Laid-Open Nos. 2006-337579 and 2007-57943 each disclose a resin-filled carrier in which a resin is filled in the voids (pores) formed in a core material, and also describe that various elements can be used as core materials. The resin-filled carriers in these Japanese Patent Laid-Open Nos. 2006-337579 and 2007-57943 undoubtedly each have an advantage that high-quality images tend to be obtained over a long period of time due to the effects such as the achieved weight reduction, but are each substantially a Mn-containing ferrite, and each suffer from problems such that the electric resistance of the carrier core material tends to be low and the variations of the magnetization and the porousness (pore formation condition) among particles tend to occur. Further, the use of a heavy metal is ineffective from the viewpoints such as the recent environmental regulation and the recent reduction of environmental load.
Japanese Patent Laid-Open No. 2008-175883 discloses a carrier for an electrophotographic developer, using a carrier core material in which the Mn content is reduced as much as possible. However, the carrier described in Japanese Patent Laid-Open No. 2008-175883 is mainly composed of Li, and suffers from a problem such that the presence of Li increases the hygroscopicity, and the charging property and the electric resistance property are largely varied depending on the use environment (temperature and humidity). In particular, when such a carrier is used as a resin-filled carrier, the specific surface area is increased due to the porousness of the carrier core material, and hence is far from satisfying the recent high demands for the reduction of the temperature and humidity dependence.
As described above, there has hitherto been demanded a resin-filled ferrite carrier for an electrophotographic developer, capable of maintaining high charging ability for a long period of time, capable of obtaining a high image quality and capable of reducing the image defects, while the advantages of the above-described resin-filled carrier are being maintained.